oem.tgz is a file used by ESXi to customize the installation. With this file you can replace default files that ESXi uses or add additional files to the install. Below are some sample files which enable support for various storage controllers. Please search the forum for other files. See this page for customizing oem.tgz for ESXi 3.5.

Updates (2010.12.23):VMware snuck in a Marvell (sky2) driver into ESXi 4.1. Read more about that here. For the Realtek 8168 NIC there are a number of drivers available. The one posted in this thread seems to solve some stability issues that some have experienced with other drivers.

If you're using the installable CD image to install ESXi, you will have to modify oem.tgz in two places. First you will find oem.tgz in the root of the install CD. ESXi 4.0 no longer user oem.tgz at the root of the CD as ESXi 3.5 did so the file won't be there by default. You can add the file and also update isolinux.cfg to include oem.tgz in the boot sequence. This copy of the file will then be used when ESXi boots from the CD to perform the installation to CD. Should you want to add additional drivers or modify simple.map/pci.ids to recognize additional hardware, then you would first need to modify this file. Otherwise the install process will not be able to recognize the hardware you wish to add support for. The second copy in on a disk dump image within the file image.tgz. For ESX 4.0 that file can be found at this path within the install.tgz file - image.tgz/image.tgz.temptar/usr/lib/vmware/installer/VMware-VMvisor-big-164009-x86_64.dd.bz2/VMware-VMvisor-big-164009-x86_64.dd. This copy of oem.tgz would be used when ESXi boots after the install. You would need to modify this copy for ESXi to boot using it after the installation has been completed.

While it is possible to modify oem.tgz in the dd image, you may find it easier to change the file after it has been installed to your system. This would make your install process look like this should you need to add hardware support:
1) Modify oem.tgz on the install CD
2) Boot the CD and install ESXi
3) After the install is complete, reboot the host with a bootable Linux CD
4) Modify oem.tgz on the Hypervisor1 partition
5) Reboot your host and ESXi will boot using the modified oem.tgz file

Update: if you're running Linux and want to automate the above process, check out the scripts here provided by Varazir. He has provided scripts for embedding oem.tgz into an install CD as well as to create a bootable flash drive with oem.tgz added. If you don't want to deal with buring a CD you can also install ESXi from a USB flash drive.

Update: A Windows script to customize the install ISO is available here.

Modifyng oem.tgz when booting from a USB flash drive

When you boot ESXi from a USB flash drive you will have copied the install dd image from image.tgz and you only have to modify oem.tgz on the Hypervisor1 (/bootbank) partition. You can either do this prior to booting ESXi or after. In the below example, ESXi will have been booted and you will then access the console. If you plan to modify oem.tgz to add hardware support, you'll have had to gathered the PCI ids for the hardware.

Instructions to modify oem.tgz if you're booting from a USB flash drive to add support for an unrecognized device.

This example assumes that you have deterimed the PCI ids for the hardware you want to add to the ESXi PCI database files and that you have been able to access the console of ESXi.